CROSS-REFERENCE TO RELATED APPLICATIONS

{0001} This application c!aims the benefit of and priority to U.S. Utility Patent

Application Ser. No, 13/834,361 filed March 15, 2013, which is hereby incorporated herein by reference in its entirety.

SUMMARY

{0001} Devices and methods for joint replacement are disclosed,

BRIEF DESCRIPTION OF THE DRAWINGS

[00023 Fl 3- 1 schematically shows a joint,

{00033 P G. 2 schematically shows a spacer for hemi-joint replacement.

[0004] FIG. 3 schematically shows an exploded view of a spacer for hemi-joint replacement positioned in a joint,

[00051 FIG. 4 schematically shows a cutting implement for shaping a bone to receive a spacer for hemi-joint replacement,

[00063 PIG- 5 schematically shows a cutting implement for shaping a bone to receive a spacer for hemi-joint replacement.

{00073 P G. 6 schematically shows an anterior view of a spacer for hemi-joint replacement in the proxima! phalangeal joint.

{0008] FIG. 7 schematically shows a media! view of a spacer for hemi-joint replacement in the proxima! pha!angea! joint.

[0009] FIG. 8 schematically shows a cutting bit and guide for shaping a bone to receive a spacer for hemi-joint replacement,

[00103 F' . 9 schematically shows two views and a cross-section of a spacer for hemi-joint replacement in the talonavicular joint.

[00113 P G, 10 schematically shows a bit for shaping a bone to receive a spacer for hemi-joint: replacement

{00123 PIGS, 11, 12 and 13 schematically show a bit, bushing and mount for shaping a bone to receive a spacer for hemi-joint replacement.

DETAILED DESCRIPTION

[0013] A spacer for joint rep!acement can be used to repiace a portion of bone in a joint. Such spacers and methods for using such spacers are described in U.S. Pat, No. 8,303,664, issued November 6, 2012, which is hereby incorporated herein by reference in its entirety. The joint is originally formed by two bones, each having a articulating surface, either of which may have some, ail or none of the related cartilage remaining. The joint schematically shown in FIG. 1 includes a first bone 101 with an articular surface 102 meeting a second bone 103 with an articular surface 104, The cartilage is not drawn separately for simplicity, but is meant to be Included in each schematically drawn bone, to the extent that cartilage remains. A patient may need to have a portion of the first bone repiaced., while the second bone remains relatively healthy with a functional articular surface, in that case, a full joint replacement, in which portions of both bones are removed, is undesirable. Instead, a hemi-joint replacement spacer can be used to repiace a portion of the first bone, while leaving the second bone entirely intact. It is desirable to leave as much healthy bone undisturbed as possible. The joint replacement spacer can be designed to interact with the natural articular surface of the second bone, or else to articulate with a prepared surface of the second bone.

[0Ο14| A wide variety of combinations of curved elements could be used to generate a y particular cut surface so as to be complementary to a spacer. A cut surface can be formed with multiple radii of curvature generated by two or more generators, or "surface-generating curves." Typically the surface-generating curves will be planar, in some embodiments, a cutting bit wili include two different radii of curvature on the cutting surface and a third radius of curvature wili be introduced by translating or rotation the bit through another curve, located on either a collar or on a cutting guide, in some embodiments a cutting burr wili generate a single radius of curvature while other curves and radii of curvature are generated by the collar, or by the guide, or by both the coiiar and the guide. One of skill in the art will recognize that many different combinations of generators are possible, and any system that generates the necessary three (or four, or more) radii of curvature would suffice. Some radii of curvature will be infinite, meaning that at least a portion of some generating curves may be fiat.

[00153 Several examples are shown schematically in the figures and are described below.

[00163 FIG. 2 schematically shows a hemi-joint replacement spacer 201. The spacer 201 has an articulating surface 202 for {a[ replacing the articular surface of a first bone, and {b) articulating with a second bone, fn reaiity, the articulating surface 202 would probably not be substantially fiat as shown, but would rather be curved to mimic the removed articular portion of the first bone. The spacer 201 also has a secondary surface 203, at ieast part of which, or in some cases ail of which, is a stabilizing surface sized and shaped to fit against a cut surface of the first bone. The cut: surface can be prepared so as to be generally complementary to the stabilizing surface. In contrast to joint im lants that are affixed to the bone by an interference fit, a surface to allow ingrowth, cement, screws, or the like, when the spacer 201 is in place on the cut: surface of the first bone, the spacer is allowed a small amount of movement relative to the bone. It is the interaction of the stabilizing surface and the cut surface of the first bone that generaliy stabilizes the spacer relative to the first bone without compieteSy immobilizing the spacer re!ative to the first bone,

[00173 HG. 2 shows that the spacer can have a first axis 204 and a second axis 205. The two axes may be perpendicular to one another, as shown to RG . 2, or they may be non-perpendicufar. One or both axes may be curved. The spacer 201 shown in FIG, 2 has a specific shape in each cross- section perpendicular to the axes. In each cross-section, the spacer 201 defines a curve 206, 207, The first curve 206 has first and second portions 208, 209 that have non-equal radii of curvature shown as j and f¾ respectively. The second curve 207 has third and fourth portions 210,. 211 that have non-equal radii of curvature shown as R _t and f¾ respectively. Although F!G, 2 shows the first and third portions having the same radius of curvature, R _¾ , in some embodiments the first and third portions may have non-equal curvatures, in the pictured embodiment,, the radius of curvature of the first and third portions,. R _lf is equal to the radius of a circular bone cutting toot, described in more detail below, FIG. 2 also shows that the second curve 207 includes a fifth portion 212, In this case, the fifth portion 212 is flat, i.e. it has zero curvature,, or an infinite radius of curvature, but in practice a fifth portion could have any radius of curvature, in this case, the spacer also includes a bump 213, which is an optional feature of the stabilizing surface. When using a spacer 201 that inc!udes a bump 213, after forming the cut surface with the

complementary radii of curvature, a cavity will also need to be formed complementary to the bump 213.

[O018| The spacer shown in FIG, 2 has a generally convex stabilizing surface, and is designed to be seated on a complementary, concave, prepared surface of the first bone. Alternatively, the stabilizing surface can be generally concave, designed to be seated on a complementary, convex, prepared surface of the first bone. Other possible shapes include the cases where either the second portion or the fourth portion is flat with no curvature (i.e., infinite radius of curvature). Also the stabilizing surface could be saddle-shaped, so that the second portion is concave while the fourth portion is convex, or vice versa, f n some saddle-shaped embodiments the radii of curvature of the second and fourth portions may be equal in magnitude,, but opposite in sign. In that case the two radii of curvature of the second and fourth portions are considered to be not equal.

[0019J As shown in FIGS. 2 and 3, the perimeter of the spacer is generally elliptical, Without altering the relationship of the various portions of the first and second curves, the perimeter could have essentially any shape in the x-y plane shown in FIG. 3, such as a circle, ova!, trapezoid, parallelogram,, kite, triangle, or any other shape that would help the space to mimic the anatomy of the replaced portion of bone.

[0020] FIG, 3 schematicall shows the spacer 301 above a bone 302 in an exploded view. The convex stabilizing surface 303 of the spacer 301 is visible along with the concave prepared surface 304 of the bone 302. The spacer is shown oriented in space by a three-dimensional Cartesian coardinate system with axes >:, y and z, and with arrows indicating rotation about each of the three axes. Rotation about the x axis is "roll," rotation about y axis is "pitch," and rotation about the z axis is "yaw," In this particular embodiment, the horizontal x and y axes are the first and second axes of spacer.

[00213 One property of the spacer 301 shown in FiG. 3 is that, when the stabilizing surface 303 is seated against the prepared surface 304 of the bone 302, the spacer is substantially prevented from yawing, i.e., rotating about the z axis. Because the first and third portions have different radii of curvature, the spacer 301 cannot yaw and remain fully seated against the bone 302; any rotation of the spacer 301 will tend to cause it to ride up off of the prepared surface 304. If the second and fourth portions had equal radii of curvature,, the spacer might be ab!e to yaw. But because the second and fourth portions are not equally curved, the aspect of the cut surface that has been shaped to match the second portion cannot also match the fourth portion. Any yawing will therefore tend to cause the spacer 301 to ride up and off of the cut surface 304. To the extent that the reapproximated joint holds the stabilizing surface 303 down onto the cut surface 304, the spacer 301 wiil be inhibited from yawing. Because the cut surface 304 will: never be perfectly complementary, it: may be possible for the spacer 301 to yaw slightly, but generally, the mating surfaces will inhibit the spacer 301 from riding up and off the surface 304, Similarly, the fact that the first and second radii of curvature are not equal will tend to prevent the spacer 301 from pitching, or rotating about the y axis. The fact that the third and fourth radii of curvature are not equal wiil tend to prevent the spacer 301 from rolling, or rotating about the x axis.

[ΟΟ223 FKS- 4 schematically shows a cutting implement, or bit, or burr, 401 useful for removing portions of bone having a defined radius of curvature. The bit is configured to be attached to a powered rotational driver, for example an air driven drill or auger, or any other source or rotational motion. The bit 401 has a rotating cutting blade. One portion 402 of the cutting blade defines a particular first radius of curvature shown as R _s . A second portion 403 of the cutting blade defines a radius of curvature shown as S¾. The implement 401 also has a collar 404, in this embodiment the collar can also have radius of curvature R3, although in other simitar

embodiments,, the collar could simply be flat. The coliar is designed to ride on a guide 405. This particular guide is genera!iy oval shaped, and is intended to be secured to the first bone, positioned directly above the portion of the first bone to be removed, The edge 406 of the guide defines a radius of curvature R _? . To form the cut surface, the bit 401 is plunged downward until the col!ar 404 contacts the curved edge 406. The rotating bit is then swept along the guide 404 so as to form the cut surface on the bone, A cross-section of the resulting surface parallel to the fortg axis of the guide wiil have radii of curvature R _s near the edge and R in the middle. A cross-section of the resulting surface para!fe! to the short axis of the guide wit! have radii of curvature and ¾ near the edge and R _¾ in the middle. Thus, the resulting cut surface will be complementary to a spacer 201 like the one shown in FiG. 2, except for the bump 213, As shown in FiG. 4, the cutting portion 402 of the bit 401 at its widest point has a width marked W which is roughiy equal to, or slightly sma!ler than, the width of the opening in the guide 405, also marked , in this embodiment,, the bit 401 would be plunged downward to meet the guide 405 and then swept back and forth in only one direction,

[0023 j As an alternative, the bit could be formed as a hemispherical cutting surface with radius of curvature _a at ait points on the cutting surface, in that case, a different part would need to generate the Rs radius of curvature. One possibility would be to make the bit have a width significantly smaiier than the inner dimension of the guide 405, Then the user would be free to sweep the bit in two dimensions over the guide. The collar 404 can then be formed with radius of curvature R _¾ to generate that radius of curvature as the bit is swept along the direction of the short axis of the guide 405.

[0024] As shown in FiG. 4, the bit 401 used in combination with the guide 405 would result in a concave prepared surface. As an alternative, the bit 401 and guide 405 couid be designed to result in a prepared surface that is convex, The radius of curvature of the collar 404 couid be inverted so that the coliar is concave with radius of curvature ¾. And likewise radius of curvature of the curved edge 406 of the guide 405 couid be inverted so as to have a convex radius of curvature R ₂ . in that case, with a concave collar and a convex guide, the resulting prepared surface of the bone would be convex. As still another alternative, both the collar and the guide could be convex, resulting in a saddle-shaped prepared surface. .As still another alternative, both the coliar and the guide could be concave, resulting in a different saddle-shape for the prepared surface,

| 02SJ FIGS. 2-4 can also be thought of as an examp!e of the more general statement of the properties of the stabilizing surface and complementary cut surface as defined by planar generating curves. In the case where the cutting surface 402, 403 of the bit 401 has two radii of curvature R _t and R ₃ and the coiiar 404 is fiat, the cutting surface defines one pianar generating curve having, in this case, two radii of curvature. That generating curve is combined with the planar generating curve defined by the curved edge of the guide 406, By combining {1} the curve defined by the cutting surface 402, 403 of the bit, and {2} the curved edge of the guide 406, a surface is created. The surface has two geometrically independent aspects, nameiy the first and second axes 204, 205. Along these geometrically independent aspects, that is, in cross-sections perpendicular to the two axes, the surface has two radii of curvature, !rt one geometrical aspect, the radius of curvature varies from j to !¾ and back to P so that the radius of curvature is non- constant, in the other aspect, the radius of curvature varies from Rj to R ₃ and back to R _t so that the radius of curvature is again non-constant in a second geometrica! aspect,

[0026} "Geometrica l!y independent aspects" is used herein to denote non-congruent axes such that every point on the surface has a unique coordinate with respect to the axes. Examples of geometrically independent aspects include perpendicular linear axes, as shown in FIG.2, non- perpendicuiar linear axes,, and curvilinear coordinate systems such as cylindrical systems in which one aspect is radial and another is azimuthai, toroidal systems in which radial distance is measured from: a circle of fixed radius, and azimuth is measured about the center of the fixed circle, toroidal systems in which the fixed radius of the toroid varies with angle for example eiiipticai, hyperbolic, o parabolic coordinate systems, etc. in the case of linear axes, a "cross-section" perpendicular to the axis is easy to visualize since it is entirely in a single plane, in the case of a coordinate system with an angular axis, at least one cross-section may be harder to visualize, since it may lie in a more compiicated surface, such as a circular, eiiipticai, parabolic or hyperboiic cylinder, rather than a simple plane as in the case of linear axes. Nonetheless, the intersection of the stabilizing surface with such a geometrical surface defines a type of cross-section and is weil-defined. in the case of curvilinear coordinates, the existence of a curve In the coordinat system may constitutes one of the generating curves of the surface,

[00273 i6. 5 schematically shows a similar bit 501 with a first radius of curvature R _; on the "edge- cutting" portion 50 of the bit 501, and a different radius R ₃ on the "plunge-cutting" portion 503 of the bit. The collar 504 also has radius of curvature R _s . This bit 501 couid be used either with a guide whos width matched the width of the widest part of the edge-cutting portion 502, in which case the plunge-cuttin portion would entirety generate the aspect of the cut surface having radius of curvature ¾ while the bit 501 was swept in only one direction in the guide. Or the bit 501 couid be used with a wider guide that: aifowed the bit 501 to be swept in a second direction, so that both the col!ar 504 and the plunge-cutting portion 503 define the aspect of the cut surface having radius of curvature ¾. in the case where the width of the guide matches the width of the bit, the cutting portion of the bit defines a generating curve with two radii of curvature, while the guide defines another generating curve with a single radius of curvature. The two planar curves combine in the same way described with respect to F!<3S. 2-4,

[00283 FIG- 6 schematically shows one embodiment of a particular spacer 601. This embodiment is particularly usefu! in a foot at the metatarsal-phalangeal joint. The articulating surface 602 of the spacer 601 articulates with the metatarsal head and the spacer replaces th proximai articular surface of the proximai phalanx. As implanted in the foot, FIG . δ presents what would foe an anterior view of the spacer 601. The opposite or inferior side of the spacer 601 has an outer edge portion 603 with a first radius of curvature ¾. The centra! portion 604 of the inferior side has a different radius of curvature ¾ in this cross-section. FIG. 7 schematically shows the same spacer 60Ϊ but in a iateral view. The articular surface 602 is sti!i visible. The edge portion 603 is also still visible showing radius of curvature j. The central por ion 604 is also stil! visible, but from the !aterai direction, the other radius of curvature, i¾, is no visible, inferior or superior views of the spacer 601 would show a generally trapezoidal shape with rounded corners,

[0029) The particular embodiment shown in FiGS. 6 and 7 includes an optional stabilizing bump 605. In addition to the stabilizing surface,, which is complementary to the prepared bone surface incorporating muitiple radii of curvature as explained above, the bump is complementa y to a cavity formed on the bone surface, The bump fits inside, but does not entirely fill, the cavity, fn this way the bump can allow for some limited motion of the spacer on the prepared surface, to the extent that the bump can move within the cavity. This is explained in more detail in U.S. Pat, No. 8,303,664, issued November 6, 2012,. which is incorporated herein by reference in its entirety.

[0030] FIG. 8 shows a bit SOI for cutting and a guide S02 for cutti g a surface in a talonavicular joint. Unlike the bits in FIGS, 4 and 5, bit 801 cannot plunge cut. The cutting surface 803 can only cut perpendicular to the rota ional axis of the bit 801. The cutting surface 803 can have end portions with radii of curvature ₃ and a centra! portion with a different radius of curvature R ₂ . The bit 801 includes an end collar 804 and a pivot collar 805. When the guide is secured to the bone and the bit 801 is seated on the guide 804, the pivot collar 805 sits in pivot groove 806 while the end collar 804 rides to the shaping groove 807. The shaping groove 807 can have end sections 808 with a radius of curvature ₃ while the centra! portion 809 can have a different radius of curvature R-„ The surface of the bone is prepared by swinging the bit 801 side to side, allowing the pivot collar 80S to remain essentially stationary in the pivot groove 806. The bit 801 moves vertically due to end coffar 804 riding over the shaping groove 807. In some cases, R _s equals F¾ so that the resulting prepared surface has a single radius of curvature around its perimeter. Because the bit 801 pivots, it necessarily sweeps out an arc. This is in contrast to the bits shown in FIGS, 4 and 5, which can be used with entirety rectilinear motion if desired, The resulting prepared surface of the bone will have differing radii of curvature in the cross-sections perpendicular to an axis, the same as described in reference to FIGS, 2, 6 and 7. But in this case, at ieast one axis is an arc, rather than a line. Such an arc is shown as a dashed arc in FIG. 8. Additionally , the because at least one axis is arcuate rather than a straight iine, the cut surface and corresponding surface of the complementary Implant wit! be stab!e against roiling about the arcuate axis evert if cross sections perpendicular to the arcuate axis are curved with only a single radius of curvature,

[00311 ' ⁿ ^e case of FIG, 8, the cutting surface SOS defines one planar generating curve. Another generating curve is the arc through which the bit SOI and its cutting surface 803 is pivoted defines another generating curve. The resulting cut shape, in the simplest case where the bit pivots in a planar, circular arc, has an outline similar to a kidney bean., or cashew nut. In a radial cross- section, that is in a vertical plane that: contains the axis of rotation of the bit. the surface has exactly the shape of the cutting surface 803, perhaps having two different radii of curvature, in an independent geometrical aspect, that is, in a cross-section Sying in a vertical circular cylinder defined by the arc through which the bit S01 is pivoted, the curve defined by the surface is fiat bottomed {an infinite radius of curvature} with rounded edges whose curvature is defined by the radius of t e bit 801 {a finite, non-zero radius of curvature}. In this way, the resulting surface can have non-constant radius of curvature in each of two geometf icaliy Independent aspects.

(0032} An additional leve! of complexity is added if further changes in curvature are added to the various aspects of the surface, As noted above, the shaping groove SO? can have muitipie portions with different curvature in a vertical plane so that the cof!ar 804 rides up and down as the bit 801 is pivoted. Additionally the shaping groove 807 can have radial curvature, so that as a user pivots the bit 801, in order to keep the collar 801 mated to the shaping groove SO?, the bit 801 must move along its axis of symmetry through the pivot groove 806. The resulting shapes can have a wide variety of curvilinear independent geometrical aspects.

{00333 ΡΙ6· 3 shows an example of an imp!ant 901 that could fit on the surface prepared by the bit: 801 and guide 802 shown in FIG. 8, FIG. 9 shows a superior view of the implant looking down on to the articu!ating surface, a lateral elevation showing the perimeter portion of the stabilizing surface with radius of curvature and a central region with a convex radius of curvature R _¾ and a cross-section in the coronal plane showing a concave radius of curvature _? , in this example, the spacer has a saddle shaped articulating surface 902. The saddie shape can be seen in that the media! view of the spacer shows the articulating surface 902 as concave, while the corona! p!ane cross-section, which gives an anterior view, shows the articulating surface 902 as convex, likewise, the stabilizing surface 903 is at least partially saddle shaped. The saddie shape can be seen in that the medial view of the spacer shows the stabilizing surface 903 as convex while the anterior view coronal-plane cross-section shows the stabilizing surface 903 as concave.

[00343 F G- 10 shows another bit 1001 for use with a pivot type of guide, similar to the guide 802 shown in FIG, 8. This bit 1001 has a more exaggerated difference between the different portions of the cutting surface. Perimeter portions 1002, 1003 are shown having radius of curvature ί¾ while the central portion 1004 of the cutting surface has radius of curvature R _; . Like the other bits shown herein, the bit 1001 is fixedly attached to a drive shaft 1005, in this example, the drive shaft protrudes 1006 from the far side of the cutting surface to allow for the attachment of a collar at the far end if desired.

[00353 ^ifl rrsany of the embodiments described herein, for example the embodiment shown in FIG. 5,. a rotating drive shaft turns the bit: while a collar or bushing of some kind surrounds the drive shaft. In some cases the collar rotates with the shaft, so that the rotating collar rides on a guide, in that case, the collar will need to be rotational!y symmetric about the axis defined by the drive shaft. In other cases, the collar or bushing will not rotate with the drive shaft, but rather the drive shaft will rotate within the collar or bushing, in that case, the collar or bushing need not be rotationaliy symmetric, or symmetric at aft.

[00361 F1<5S. 11, 12 and 13 schematically show a different combination of a bit 1101, a bushing 1102 and a mount 1103 to be used together to form a saddle shaped prepa ed surface on the bone. The bit 1101 is a side-cutting bit like those of FIGS, 8 and 10, The bit HOI includes perimeter cutting portions 1104, 1105 defining a radius of curvature ¾ and a central cutting portion 1106 defining a radius of curvature R . The bit also includes a drive shaft 1107, Unlike the bits in FiGS. 8 and 10, bit 1101 has no protruding shaft on the far end of the bit from the drive shaft, The bit is designed without a built-in collar or bushing,

[0037] instead, the bit 1101 is designed to spin in bushing 1102. FIG, 12 schematically shows three views of the bushing 1102, The bushing 1102 is generally arcuate in shape and an arc through the center of the bushing, shown as a dashed line, defines a radius of curvature ₃ . The bushing 1102 defines a channel 110S A channel 1108 through the bushing is sized to receive the drive shaft 1107 of the bit 1101.

[00381 FIG. 13 schematically shows a mount 1103 for use with the bit ^" 1101 and bushing 1102, Clockwise from top left, FIG. 13 shows four views of the mount 1103: a top view; an angled perspective view showing, top, side and front; a side view; and a front view. The mount 1103 is to be attached to the bone where the surface will be prepared, and may be affixed, for example with screws or the like through the anchoring holes 1109, The mount defines a bushing slot 1110, sized and shaped to receive the bushing 1102, having the same arcuate shape and radius of curvature of the bushing 1102. in use, the bushing 1102 can siide side to side in the mount 1103. in some embodiments the rotating bit 1101 can slide front to back through the bushing, and the mount 1103 is deep enough in comparison to the bit 1101 that there is room to move the bit forward and backward, in other embodiments, the bit 1101 is roughly the same depth as the depth of the mount: 1103 so that the bit only moves side to side. The mount can also define an entry slot 1111 through which the bit can be introduced into the cutting position inside the mount 1103.

[00391 T is set-up shown in FIGS, 11-13, !tke the guide and bit in FiG, 8, can b used to create a stabilizing surface with curviiinear independent geometricai aspects, or it can be simpiified to keep the independent geometrical aspects essentiai!y recta gular. As shown, the bushing 1102 rides in bushing siot 1110 which is cut in to a curved front waii of the mount 1103. The bit 1101 must rotate as the bushing 1102 moves in the slot 1110 in order to keep the bushing 1102 fuiiy seated, in some embodiments, the bushing 1102 may be constrained to so move, for exampie by a mating surface with the mount 1103, e.g, a tongue and groove or similar arrangement. Aiternative!y, if the mount 1103 had a fiat front, the bit 1101 would not have to rotate at ail to keep the bushing 1102 fully seated, and the resulting surface ouid have rectilinear independent geometrical aspects,

[0040J to any of the preceding examples, or in other embodiments, a kit comprising a bone- cutting bit and a guide can comprise a kit for cutting a prepared surface on a bone so as to be compiementary to a particular spacer, A kit couid inciude the spacer, A kit couid inciude bushings and or coiiars necessary to generate a particular radius of curvature.

[0041] in any of the preceding examples, or in other embodiments, a kit comprising a bone- cutting bit and a guide can comprise a kit for cutting a prepared surface on a bone so as to be compiementary to a particular spacer. A kit couid inciude the spacer. A kit couid inciude bushings and or coiiars necessary to generate a particular radius of curvature,

[00423 The joint to be partiai!y replaced can be any synovial joint in the body. The discSosed devices and methods may be especia!iy beneficial in smail joints such ss in the extremities. The joint can be in the foot, for exampie, the tibiotafar, taiooavicufar, metatarsal phaiangeai, metatarsal: tarsal:, navicuiarcuneiforrrt, calcaneal: cuboid, subtalar, and interphaiangeal {distal and proximai) joints, The joint can also be in the hand or wrist, for exampie, the carpometacarpal joint of the thumb, the scapho-trapezio-trapezoid fSTTi, the metacsrpal-phsiangesi joint of the thumb, and the proximai interpha!angeal joints. Eifoow, shoulder and knee joints may also tae we!l suited to the disclosed methods and devices.

[0043| The combination of two or more generating curves with non-constant curvature can result in a wide variety of different surfaces and shapes, aliowing the spacer and compiementary surface to mimic a wide variety of bones and to function in a wide variety of joints. The resulting complementary surfaces on the bone and the spacer range from simple to complex, in various embodiments, the desired stability can be achieved with an implant whose stabilizing surface has at !east three different curvatures distributed across two different axes or geometrically independent: aspects. It should be understood that in general the goal of preparing a bone with such a surface is to allow the prepared surface to mate with a complementary stabilizing surface on a spacer, although one could prepare a bone with a cut surface for another purpose,

10044} Any of the spacers described herein can be made of a variety of biocompatible materia!s, including pyrolytic carbon. In particular, the articulating and stabilizing surfaces may be formed entirely of pyrolytic carbon, as may the bump, if the spacer includes one. Pyroiytic carbon has been used as an implant: material for several decades in artificial heart valves and artificial joint implants. When used within a joint, the materia! exhibits extreme biocompatibility,. surpassing common implant metals such as stainless steels, titanium alloys, ceramics, and cobalt chrome alloys, This extreme biocompatibility allows the body to act in two advantageous ways. First, pyrolytic carbon can transmit sliding motion under load to synovial joint surfaces white allowing the surfaces to remain healthy and functional ove prolonged periods of time. Its biocompatibility against articular cartilage clinically surpasses all presently-known common implant metals, This property provides prolonged load bearing contact between the joint spacer and the remaining cartilage surface, The second advantage of pyrolytic carbon is its reaction in the presence of newly formed bone surfaces, such as are created when the end of a diseased bone is resected. In this case, pyrolytic carbon appears to encourage the formation of a new load transfer surface that responds favorably to small induced motions while transmitting joint toads. This property leads to the advantageous element of the claimed joint spacer, namely the absence of the need for rigid fixation between the spacer and the bone.

{00453 ^A spacer useful in hemi-joint replacement for replacing a removed portion of a first bone and articulating with a second bone can have an articulating surface and a stabilizing surface. The articulating surface can be sized and shaped to articulate with an articular surface of the second bone, The stabilizing surface can be sized and shaped to conform to a cut surface of the first bone. The spacer can define a first axis and a second axis not parallel to the first axis. The first and second axes may or may not be perpendicular. One or both axes can be lines or curves, in a cross- section of the spacer perpendicular to the first axis, the stabilizing surface defines a first curve. In a cross-section of the spacer perpendicular to the second axis, the stabilizing surface defines a second curve. The first curve can have a first portion with a first radius of curvature and a second portion with a second radius of curvature, where the first and second radii of curvature may be unequal. The second curve can have a third portion with a third radius of curvature and a fourth portion with a fourth radius of curvature, where the third and Fourth radii of curvature may be unequal. Moreover, the second and fourth radii of curvature may be unequai as weii.

[0046] The first and third radii of curvature may be equa! to each other, and equal to the radius of curvature of a curved cutting blade of a bone shaving instrument. The second curve can further include a fifth portion with a fifth radius of curvature. The fifth radius of curvature can be unequal to both the third and fourth radii of curvature. The fifth radius of curvature could be, for example, infinite meaning that the fifth portion is substantially flat.

|004?3 The stabilizing surface may include a bump, or protrusion, designed to sit inside a cavity defined by the prepared cut surface of the first bone. The bump would partialiy, but not entirely, fil! the cavity so that, when the stabilizing surface i ful!y seated on the cut surface, the bump would protrude into the cavity. The movement of spacer on the cut surface could then be limited by the amount of movement possible by the bump in the cavity. Such arrangements are described in U.S. Pat. No, 8,303,664, which is incorporated herein by reference.

[004SJ The stabilizing surface may be convex, concave, saddle-shaped, or a more complex combination of many radii of curvature. The perimeter of the spacer may be a circle, oval eiiipse, torus, trapezoid, para!fe!ogram, kite, triangle, or any other shape that would help the spacer to mimic the anatomy of the replaced portion of bone. The exterior of the spacer may consist entirely, or essentially, of the articulating surface and the stabilizing, surface, and no other, or essentia!iy no other, surfaces. The first: curve may consist entirely, or essentially, of the first: and second portions, and may have no other, or essentialiy no other, portions. The same may be true for the second curve, Either the first or second axis, or both, may be curved.

[O049j The spacer can include pyroiytic carbon, can consist essentialiy of pyroiytic carbon, or can consist entirely of pyroiytic carbon. In particular, the articulating surface and/or the stabilizing surface may be formed of pyroiytic carbon.

[00503 Any such spacers may be included in a kit along with a bone-cutting bit: having a cutting portion and a guide sized and shaped to guide the bone cutting bit. The bone-cutting bit and the guide can be sized and shaped such that, when the bone-cuttin bit is guided by the guide, the cutting portion sweeps out a surface complimentary to the stabilizing surface of the spacer. A profile of the cutting portion can define a curve at least a portion of which has the first: radius of curvature and a profile of the guide can define a curve at least a portion of which has the fourth radius of curvature, A profi!e of the cutting portion can be congruent to the first curve.

[0051| A joint that includes a first bone having a first articular surface and a second bone having a second articular surface can be distracted. The first bone can be prepared in at least two steps. First, the first bone may be prepared by removing the first: articular surface, thereby creating a cut: surface on the first bone, the cut surface defining a first axis and a second axis not paraiiei to the first axis. Second, the first bone may be prepared by shaping the cut surface so that: in a cross- section of the cut surface perpendicuiar to the first axis, the cut surface defines a first curve including a first portion with a first radius of curvature, and a second portion with a second radius of curvature not equa! to the first radius of curvature; in a cross-section of the cut surface perpendicular to the second axis, the cut surface defines a second curve inciuding a third portion with a third radius of curvature, and a fourth portion with a fourth radius of curvature not equa! to the third radius of curvature; and the second and fourth radii of curvature are not equal. A spacer may be placed against the cut surface of the first bone, the spacer having an articulating surface sized and shaped to articulate with an articular surface of the second bone, and a stabilizing surface sized and shaped to be substantially complementary to the cut surface, The joint may be reapproximated such that the articuiating surface of the spacer articulates with the second articuiar surface of the second bone, and the stabilising surface of the spacer fuiiy seats against the cut surface of the first bone.

£0052] A joint that includes a first bone having a first articuiar surface and a second bone having a second articuiar surface can be distracted. A spacer may be provided,, the spacer having an articulating surface sized and shaped to articulate with an articular surface of the second bone, and a stabilizing surface a predetermined shape. The first bone can be prepared by removing the first articular surface of the first bone thereby creating a cut surface sized and shaped to be substantially corn pie frtentary to the stabilizing surface. The spacer can be placed against the cut surface of the first bone with the stabiiizing surface facing the compiementary cut: surface of the first bone. The joint can be reapproximated such that the articulating surface of the spacer articulates with the second articuiar surface of the second bone, and the stabilizing surface of the spacer fuiiy seats against the cut surface of the first bone. The stabiiizing surface can be defined by a combination of at least two surface-generating curves such that: two geometrically independent: aspects of the stabilizing surface are defined by non-constant curvature.

[0053] A spacer useful in hemi-joint re lacement for repla ing a removed portion of a first bone and articuiating with a second bone can have an articuiating surface and a stabilising surface. The articulating surface can be sized and shaped to articuiate with an articuiar surface of the second bone. The stabiiizing surface can have a shape defined by a combination of at ieast two surface- generating curves such that two geometrically independent aspects of the stabilizing surface are defined by non-constant curvature. The two geometrically independent aspects can be rectilinear axes, which may or may not be perpendicular to one another. The two geometrically independent aspects can also be defined fay a curvilinear coordinate system. Curvilinear coordinate systems include, for example polar, bipolar, parabolic, elliptic, hyperboiic, circular cylindrical, parabolic cylindrical, elliptic cylindrical, hyperboiic cylindrical, spherical, obiate spheroidal, prolate spheroidal, and toroidal coordinates. Any system in which geometrically independent aspects are defined in space can suffice.

[0054] Any such spacers may be included in a kit along with a bone-cutting it having a cutting portion and a guide sized and shaped to guide the bone cutting bit. The bone-cutting bit and the guide can be sized and shaped such that, when the bone-cutting bit is guided by the guide, the cutting portion sweeps out a surface complimentary to the stabilizing surface of the spacer, A profile of the cutting portion can define one of the at least two surface-generating curves and a profile of the guide can define another of the at least two su face-generating curves.